This invention relates generally to electrophotographic printing, and more particularly concerns an optical system employing a half-tone screen which minimizes moire patterns.
Conventional screening methods employed in electrophotographic printing machines produce the effect of tone gradation by means of dot size variations. In the highlight regions, the dots are small and increase in size through the intermediate shades until they merge together in the shadow regions. At the highlight end of the tone scale, there is complete whiteness, while, at the shadow end, there is nearly solid blackness. This type of pattern can be reproduced in an electrophotographic printing machine through the use of dot screens.
Many techniques have been developed to improve half-tone reproductions. In graphic arts, moire patterns are minimized by changing the screen orientation between successive single color half-tone patterns. With the advent of colored electrophotographic printing, screening techniques have been employed to improve copy quality. Multi-colored electrophotographic printing is similar to black and white printing. The process of black and white electrophotographic printing is described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942. In multi-color electrophotographic printing, the light image is filtered producing successive single color light images of the original document. These colored light images expose a charged photoconductive surface to create successive single color electrostatic latent images thereon. Each single color electrostatic latent image is developed with toner particles complementary in color to the color of the filtered light image. The toner powder images are transferred from the electrostatic latent image to a sheet of support material, in superimposed registration with one another. In this manner, a multi-layered toner powder image is formed on the sheet of support material. This multi-layered toner powder image is then permanently affixed to the sheet of support material forming a color copy. Half-tone screens are employed in multi-color electrophotographic printing to enhance the copy being reproduced thereby. Generally, the screen is interposed into the optical light path and successive single color light images are transmitted therethrough onto the charged photoconductive surface forming an image comprising a plurality of dots. As described in co-pending application Ser. No. 541,748 filed in 1975, now U.S. Pat. No. 4,012,137 the screen may be rotated through successive angles between each single color light image. Thus, for the first single color light image, the screen is at a 15.degree. angle relative to the horizontal. For the second light image, the screen is at a 45.degree. angle relative to the horizontal, and for the final light image, the screen is at a 75.degree. angle relative to the horizontal. However, in a system of this type, the screen must be rotated between successive single color light images in order to minimize the formation of moire patterns which severely degradate copy quality.
U.S. Pat. No. Re. 26,058 issued to Archer in 1966, discloses a half-tone screen comprising a plurality of differently colored dots thereon. The dot patterns are rotated relative to one another by 30.degree.. Thus, one set of dots is positioned at 15.degree. relative to the bottom edge of the screen base, the second set of dots being positioned at 45.degree. relative to the base edge of the base, and the third set of dots being positioned at 75.degree. relative to the bottom edge of the base. As described therein, the screen is employed in color photography for making direct, color-separation, half-tone negatives.
Accordingly, it is a primary object of the present invention to improve multi-color electrophotographic printing by employing a stationary screen in the optical system to obtain pictorial quality copies therefrom.